Picture tube restorer



March 25, 1958 u. L. NISSEL ET AL PICTURE TUBE RESTORER 2 Sheets-Sheet 1Filed July 16, 1956 NTOR )Z'ssed,

AA A i Z March 1958 U. L. NlSSEL ET AL 2,828,446

PICTURE TUBE RESTORER Filed July 16, 1956 2 Sheets-Sheet 2 F.\ ii}?- If70-1 Z4 me Q ATTORNEY }Uniterroronn "ruse nnsronnn Application July 16,1956, Serial No. 597,929

6 Claims. (Cl. 315-51) This invention relates to a cathode ray tuberejuvenator. More particularly this invention relates to a device whichenables the picture tube of a television receiver to be continued insatisfactory service under circumstances which previously would haverequired the installation of a new picture tube. The picture tube is arelatively expensive component of a television receiver, and picturetube 'orightener have achieved widespread public acceptance as one meansof prolonging the service life of cathode ray tubes whose light outputhas decreased. The ordinary brightener is usually nothing more than atransformer which elevates the voltage applied to the caLhode ray tubefilament, thereby elevating the filament temperature and maintainingelectron emission from the somewhat exhausted cathode. The elevatedfilament temperature maintains cathode emission by supplying sufiicientenergy to lowenergy electrons in the cathode structure to allow them toovercome the barrier potential at the surface of the cathode. Theseliberated electrons would never escape the cathode under the conditionsof normal filament voltage and temperature. Cathode ray tubes aremulti-electrode electronic devices which are subject to failures ofseveral kinds. The ordinary tube brightener is able to rectify only onetype of failure, namely that of decreased picture brightness due to theonset of cathode exhaustion or gassiness in the cathode region of theelectron gun. This invention, however, enables the picture tube to becontinued in satisfactory service not only when the tube ischaracterized by a low emission or gassy condition resulting in adecrease of picture brightness, but also in the presence of tubefailures due to other common faults, such as, by way of example: aninternally open cathode circuit; an internal control grid to cathodeshort circuit; an internally open cathode circuit combined with aninternal control grid to cathode short circuit; an internally opencontrol electrode circuit; r

an internal filament to cathode short circuit. In short, this inventionprovides a device which will cure all of the commonly occurring picturetube failures and restore the picture tube to approximately ninetypercent of its performance immediately preceding the development of thedefect. Prior to this invention no known auxiliary equipment has everbeen devised which could rectify any one of the first four cited picturetube defects.

Accordingly, it is an object of this invention to provide novelapparatus which is capable of restoring picture tubes to service whichheretofore have been considered to be beyond the possibility ofcontinued use.

Another object of this invention is to provide-novel apparatus forrejuvenating the picture tube of television receivers which is small,non-power consuming, easily installed, and inexpensive relative to thecost of such picture tube.

Another object of this invention is to provide novel apparatus forrejuvenating the picture tube of television receivers which may be usedwith either parallel or seriesstring filament circuits.

States Patent Y ce j Yet another object of this invention is to providenovel apparatus for rejuvenating the picture tube of televisionreceivers which contains as an integral part thereof a variable gainvideo signal amplifier for increasing the signal drive to the picturetube.

Still another object of this invention is to provide novel apparatus forrejuvenating the picture tube to television receivers which providesmeans for variably controlling the brightness and contrast levels of theviewed picture.

Other objects of the invention will in part be described and in partbecome apparent as the following specification is read in conjunctionwith the drawings, wherein:

Fig. l is a partial schematic and partial sectional representation ofone embodiment of the invention illustrat-, ing the internal connectionsand the manner of connection into a television receiver.

Fig. 2 is a partial schematic and partial sectional representation of asecond embodiment of the invention containing as an integral part avideo signal amplifier.

Figures 3 to 8 inclusive illustrate methods of interconnecting the screwlug terminals of the picture tube rejuvenator to correct some commonlyoccurring faults,

and are illustrative of typical applications of the invention. Thesecommon faults are:

Fig. 3an internally open cathode condition.

Fig. 4-an internal control electrode to cathode shortcircuit.

Fig. 5an internally open cathode condition combined with an internalcontrol electrode to cathode short circuit.

Fig. 6--an internal filament to cathode short circuit.

Fig. 7-2. low emission or gassy condition.

Fig. 8-an internally open control electrode circuit.

The several figures have been portrayed with a view toward maintainingvisual clarity insofar as possible, and are not to be construed asnecessarily representative of an actual physical embodiment of theinvention.

In the several figures like numerals are used to denote like elements.

Before explaining in detail the particular methods employed by theinvention for correcting the several defects already indicated, a briefdiscussion of the gen eral method of attacking the problems posed willbe conducive to a more thorough understanding of the invention. Allcathode ray tubes utilized as picture tubes in television receiverscontain certain common features in their electron gun structure. Forexample, all such tubes contain .a filament for heating the cathode, acathode for emitting electrons, a control electrode normally biasednegative to the cathode for supplying a point source of electrons andcontrolling the electron beam intensity, and an electrode known commonlyas the first accelerating anode or screen grid which is maintained at apositive potential relative to the cathode for imparting a largevelocity to the electron beam in the direction of the viewing screen.Additionally, the electron gun may utilize a focusing electrodemaintained at a potential positive to the first ac celerating electrode,but of lesser magnitude than the potential of the second anode, whosepurpose is to focus the electron beam to a small cross-section where itimpinges on the viewing screen. The use of such an electrode is termedelectrostatic focusing. Alternatively, focusing may be accomplished bythe use of an external focus coil having a current flowing therethroughand mounted on the tube neck rearward of the deflection yoke. Theemployment of such .coil is termed magnetic focusing. Theinvention canbe' advantageously used with picture tubes employing either type offocusing.-

The general principle uti1ized.by -'the invention isthat the electrodesof the electron gun structure are not limited in their use to theaforementioned functions, but each electrode can by judiciousinterconnection combined with suitable circuit modification be made toperform functions normally reserved to other of the electrodes. Should,for instance, the normal control electrode become unusable as such, itsfunction may be assumed for example by the first accelerating electrode.

Turning now to a detailed examination of the invention and its method ofoperation consider first Fig. 1. There is shown a television chassis 125upon which is mounted a cathode ray tube or picture tube 110 having anelectron gun structure comprising a filament 124, a cathode 123, acontrol electrode 122, a first accelerating electrode 121, and afocusing electrode 120. These electrodes are connected by lead wires topins of a standard six contact male plug 70 attached to the pictu'retube at its narrower extremity. The several pins of the plug 70 areconventionally connected as follows: pin 70-2 connects to the normalcontrol electrode 122; pin 70-1 connects to one side of filament 124,while pin 70-12 connects to the other side of said filament; pin 70-11connects to the cathode electrode 123; pin 70-10 connects to the firstaccelerating electrode or screen grid 121; and pin 70-6 connects to thefocusing electrode 120. Emanating from chassis 125 is a cable 102containing six wires and terminating in a six contact female plug 40which normally mates with plug 70 on the picture tube 110. The six wirescontained in cable 102 communicate within the chassis 125 to potentialand signal sources suitable for properly energizing the variouselectrodes of the picture tube 110. As necessarily dictated by themating arrangement of plugs 40 and 70, contacts 40-1 and 40-12 connectwith a pair of wires in cable 102 which carry filament potential,normally 6.3 volts A. C.; contact 40-2 carries the video signal; contact40-11 supplies the normal cathode reference potential; contact 43-10supplies a positive potential for controlling initial acceleration ofthe electron beam; and contact 40-6 supplies the potential required bythe focusing electrode. When utilizing the invention, the plug 40 isremoved from its normal engagement with plug 70 and is mated with maleplug 50 of the picture tube rejuvenator. Plug 70 of the picture tube isthen engaged by female plug 60 of the picture tube rejuvenator. Plugs 50and 60 of the rejuvenator bear a one-to-one pin placement correspondenceto plugs 70 and 40 respectively. The rejuvenator, except for theinterconnecting plug 60 and its associated cable 101, is enclosed in ahousing 100 which may be formed from plastic or metal or other suitablyrigid material. The male plug 50 is mounted in the end of the housing100 opposite the end where cable 101 emerges. Pin 50-6 of plug 50connects to contact 60-6 of plug 60 via cable 101.

Pin 50-10 connects to one end of variable resistor 11, which maytypically have a resistance value of to 30 megohms. The movable contactof variable resistor 11 connects to one end of a fixed limiting resistor20, which typically may have a value of 200 kilohms, the other end ofwhich terminates at screw lug 7 of terminal board 10, and also atcontact 60-10 of plug 60. Variable resistor 11 may be convenientlymounted directly to the housing 100 with the movable contact shaftprojecting therethrough to provide accessability for adjustment of thepotential applied to the first ac- 'celerating electrode 121 of picturetube 110. The

function of resistor is to provide a minimum value of series resistanceas a safety measure to prevent application of the full availablepotential to the first accelcrating electrode. Pin 50-2 which carriesthe video signal connects to screw lug 6 of terminal board 10. Theremaining pair of screw lugs 8 and 9 of terminal board 10cpnnectrespectively,to'contacts 60-2 and 60-11 of plug 60 via cable 101.Pins 50-1 and 50-12 of plug 50 which receive the normal filament voltagefrom the chassis 125 via mating plug 40 connect to the poles of adouble-pole single-throw switch 29. The switch contact whichcommunicates with pin 50-1 of plug 50 through one pole of switch 29connects to one end of primary winding 15 of an isolation transformer13. The switch contact which communicates with pin 50-12 of plug 50through the other pole of switch 29 connects to a tap on primary winding15 such that a boosted voltage appears across the secondary winding 14.The ends of primary winding 15 connect to screw lugs 31 and 32 of aterminal board 34. The switch 29 is illustrated in the closed positionwhich corresponds to operation with a parallel filament televisionreceiver and may also be directly mounted to the housing with its shaftextending therethrough. However, the rejuvenator may be operated withseries-string filaments by opening switch 29 and connecting screw lugs31 and 32 of terminal board 34 to a source of volt A. C. line voltage,which is available from chassis 125. This is the only change needed forseries-string circuits. The turns ratio of full primary 15 to fullsecondary 14 is such that the proper voltage step-down is obtained forseries filament operation. Pin 50-11 of plug 50, the normal cathodeconnection, connects to remaining screw lug 33 of terminal board 34. Oneend of the secondary winding 14 of isolation and boost transformer 13connects to contact 60-1 of plug 60 via cable 101 through negativetemperature coefficient protective resistor 12 which may typicallyexhibit 1 to 4 ohms resistance when cold, and /2 ohm resistance whenheated by filament current. Resistor 12 prevents burnout of the filament124 of picture tube 110 when energizing voltage is first applied to thecold filament. The other filament contact of plug 60, namely contact60-12 is connected via cable 101 to screw lug 1 of a terminal board 5which supports three additional screw lugs 2, 3 and 4. The screw lugs 2,3, and 4 connect to taps on secondary winding 14 and provide a choice ofdifferent filament voltages for application to the picture tube bystrapping one of these lugs to screw lug 1. The voltages available atlugs 2, 3 and 4 can be for example 8 volts, ,11 volts, and 13 voltsrespectively. The choice of filament voltage will, of course, bedictated by the particular fault to be corrected, as will besubsequently shown. As a matter of preference, the terminal boards 34,5, and 10 may be eliminated, and the internal leads terminating at thescrew lugs thereon may be suitably color-coded and brought through anaperture in the housing 100. Although a particular configurationembodying the invention has been illustrated, the scope of the inventionis not intended to be limited by values of voltage or resistance statedmerely by way of illustration.

Turning now to an examination of the methods of interconnecting thescrew lugs of the several terminal boards 34,5, and 10 to cureparticular defects occurring in the picture tube, consider first Fig. 3and Fig. 1 together. Fig 3 illustrates the interconnection of lugsrequired to cure an internally open circuited cathode connection. Screwlugs 6 and 8 of terminal board 10 are strapped together by a jumper wirethereby effecting the interconnection of pin 50-2 of plug 50 and contact60-2 of plug 10. This connection permits the video signal from chassisto be applied to the control electrode 122 of picture tube 110. Screwlug 33 of terminalboard 34 is strapped to screw lug 1 of terminal board5 thereby applying the normal cathode bias supplied from chassis 125 viapin 50-11 of plug 50 to the'filament 124 of picture tube 110 via contact60-12 of plug '60. The filament now serves as both filament and cathode.Lug 9 of terminal board'10 which normally connects to the cathode '123of picture 5 tube 110 via contact 60-11 of plug 60 remains uncon nected.Lug 1 of terminal board 5 is also strapped to lug 4 thus applying ahigher than normal voltage of 13 volts to the filament 124. Theapplication of this higher voltage heats the filament 124 to a muchhigher than normal temperature and provides tight electron couplingbetween the filament 124 and the cathode 123 whereby they act as oneunit. Variable resistor 11 should be set for minimum resistance so thatthe maximum potential may be applied from pin 50-10 of plug 50 to thefirst accelerating electrode 121 of picture tube 110 via contact 60-10of plug 60. If the viewing screen brightness is too great, someresistance may be inserted by rotating the movable arm of variableresistor 11. Positive potential is, of course, applied to the focusingelectrode 120 via the before-described connection between pin 50-6 ofplug 50 and contact 60-6 of plug 60.

Fig. 4 considered together with Fig. 1 illustrates the interconnectionof lugs required to cure an internal control electrode to cathode shortcircuit. Lugs 8 and 9 of terminal board 10 are strapped together therebyconnecting the control electrode 122 to the cathode 123 and securing theshort circuitbetween them. By so doing, any variation of viewing screenbrilliance due to changing resistance between these electrodes iseliminated. The control electrode 122 is no longer used as either asignal or bias electrode, and in fact retains no electrical function.The strapping together of lugs 6 and 7 of terminal board 10 permits thevideo signal present on pin 50-2 of plug 50 to be applied to the firstaccelerating electrode 121 via contact 60-10 of plug 60. This connectionconverts the electrode 121 into a combination accelerating and controlelectrode since the positive potential present on pin 50-10 is alsoapplied to electrode 121 via contact 60-10 of plug 60 and variableresistor 11. The direct potential applied to electrode 121 can beadjusted through appropriate setting of the movable contact of variableresistor 11. This adjustment is best made while viewing the screen ofthe picture tube and regulating variable resistor 11 until properbrilliance and contrast is obtained. Generally, when the rejuvenator isconnected in this fashion a decrease of both brilliance and contrastwill be observed. The brilliance decrease is due to the lower value ofaccelerating potential applied to electrode 121, while the decrease incontrast is due to the higher velocity of the electron beam at the pointof control. These'deficiencies, which may occur when the embodiment ofthe invention shown in Fig. 1 is utilized, can be completely overcome byemployment of the embodiment of the invention illustrated in Fig. 2,which will be described in detail subsequently. Briefly, however, theembodiment of the invention shown in Fig. 2 incorporates a high-gainwide-band conventional video amplifier for increasing the video signallevel so that full contrast may be obtained. This increased video levelpermits full accelerating potential to be applied to electrode 121,hence restoring the brilliance. Returning now to an examination of Figs.1 and 4, it is seen that lug 33 f terminal board 34 is strapped to lug 9of terminal board 10, thus applying normal bias from chassis 125 to thecathode 123 of picture tube 110 via contact 60-11 of plug 60. Lug 1 ofterminal board is strapped to either lug 2 or lug 3 (illustrated asstrapped to lug 2) thus applying 8 volts or 11 volts to the filament 124depending upon the condition of the picture tube. Positive potential isapplied to the focusing electrode in the manner already described inconnection with Figs. 1 and 3.

Fig. 5 considered together with Fig. 1 illustrates the interconnectionof lugs required to cure an internally open cathode circuit combinedwith a control electrode to cathode short circuit. Lug 8 of terminalboard is strapped to -lug 1' of terminal board 5 and lug3l3 of terminalboard 34, thereby connecting the normal control electrode 122 to thefilament 124 and applying the normal cathode bias from chassis 125thereto. The cathode 123 of picture tube cannot be physically connectedto any other point due to the internal open circuit and therefore lug 9of terminal board 10 is not connected to any other lug. However, bystrapping together lugs 1 and 4 of terminal board 5, a potential of 13volts is applied to filament 124 resulting in a markedly elevatedfilament temperature and providing tight electron coupling between thecathode 123 and the filament 124. The cathode 123, filament 124 andnormal control electrode 122 now act commonly as the cathode. Thestrapping together of lugs 6 and 7 of terminal board 5 results in thesame type of operation as already discussed in connection with Figs. 1and 4 illustrating the interconnections for curing the control electrodeto cathode short circuit condition, to which reference should be had fora detailed explanation. Briefly, however, the-first accelerating electrode 1.21 assumes the functions of both the accelerating and controlelectrodes. Variable resistor 11 must be adjusted for proper brillianceand contrast, and possibly the embodiment of the invention illustratedin Fig. 2 need be utilized to restore full performance to the picturetube.

Fig. 6 considered together with Fig. 1 illustrates the interconnectionof lugs required to cure a filament to cathode short circuit. Lug 9 ofterminal board 10 is strapped to lug 1 of terminal board 5 and lug 33 ofterminal board 34, whereby normal bias is applied from chassis tocathode 123 of picture tube 110 and the short circuit between filament124 and cathode 123 is secured, thus preventing variations in theshorting resistance between the filament 124 and cathode 123 fromcausing erratic viewing conditions. Strapping of lugs 1 and 2 ofterminal board 5 provides voltage to the filament 124. When a filamentto cathode short circuit is combined with a low emission or gassycondition it may be necessary to provide a higher filament voltage, suchas 11 volts by alternatively strapping lug 1 to lug 3 of terminal board5. In an extreme case it may be necessary to provide 13 volts to thefilament by strapping lugs 1 and 4. The video signal is supplied fromchassis 125 to the control electrode 122 of picture tube 110 via thestrapping of lugs 6 and 8 of terminal board 5 as previously described inconnection with Figs. 1 and 3. The focusing and first acceleratingelectrode potentials are also supplied as described in connection withthe same figures.

The lug strapping arrangement shown in Fig. 7 illustrates theinterconnections for overcoming merely a low emission or gassy conditionand is substantially the same as that illustrated in Fig. 6, whichrelates to a filament to cathode short circuit. The only difference isthat the filament 124 is not connected to the cathode 123 since nofilament to cathode short circuit is presupposed. Lug 1 of terminalboard 5 is not therefore connected to lug 9 of terminal board 10. Asalready pointed out in connection with Fig. 6, lug 1 of terminal board 5is normally strapped to lug 3' and in an extreme case of low emission orgassiness, it would be strapped to lug 4.

The lug strapping arrangement shown in Fig. 8 illustrates theinterconnections for overcoming an internally open control electrodecircuit and is substantially the same as that illustrated in Fig. 4,which relates to a control electrode to cathode short circuit. The onlydifference is that the control electrode 122 cannotbe physicallyconnected to any other electrode due to the internal open circuit andtherefore lugs 8 and 9 of terminal board 10 are not strapped together.In all other respects the circuit operation determined by the strappingarrangement of Fig. 8 is identical to that of Fig. 4. Of course, log 1of terminal board 5 may of necessity have tobe strapped to lug 3 or 4rather than lug 2 should a low emission or gassy condition be alsopresent.

Turning now to an examination of the embodiment of the invention shownin Fig. 2, the utilization of which has already been referred to inconnection with Fig. ;4, it will be seen that it differs from Fig. 1only by the inclusion of i a high-gain wide-band video amplifierdesignated generally as St). This video amplifier is of conventionaldesign utilizing resistance-capacitance coupling and combined shunt andseries peaking circuits, A bypassed potentiometer 86, which may have avalue of 1 kilohm, included in the cathode lead of the output section ofamplifier valve 89 provides gain control and some high frequencycompensation of the video signal, and hence contrast control. The valueof the by-pass capacitor 91 may be 470 micro-microfarads. Positive Bvoltage is supplied to the anodes of the valve 89, typically a type12AU7 vacuum tube, via lead 83 and resistors 92, 93, and shunt peakingcoil 94 from pin 50-19 of plug 50, the source of potential foraccelerating electrode 121 of picture tube 116. Resistors 92 and 93 maybe 3300 ohms and 4300 ohms respectively, and coil 94 may have a value ofto 50 microhenries. Leads; 81 and 82 which connect respectively to pins50-1and 59-12 of plug 50 supply normal filament voltage to the filamentof video amplifier valve 89, lead 81 also serving as the negative Bsupply conductor. A lead 84 connects lug 19 of terminal board to theinput circuit of the amplifier valve 89 through a capacitor 95 andresistor 96 which may typically have values of .05 microfarad and lmegohm; lead 85 connects lug 18 of terminal board 10 to the outputcircuit of the video amplifier comprising capacitors 97, 9 8, and seriespeaking coil 99 which may respectively have values of .05 microfarad, 47micro-microfarads, and 5 to 50 microhenries. The interstage couplingnetwork comprising capacitor 87 and resistor 88 may have values of .95microiarad and l megohm. Only in those cases, previously described, inwhich the first accelerating electrode 121 also assumes the function ofa control electrode should the lugs of terminal board 10 be strapped asshown, otherwise the lugs 18 and 19 are not used and the luginterconnections are the same as described in connection with theembodiment of Fig. l. The strapping of lugs 6 and 19 of terminal board10 provides a connection whereby the video signal from pin 502 isapplied to the input of the video amplifier 80. The strapping of lugs 18and 7 of terminal board 10 provides the connection whereby the amplifiedvideo signal appearing at the output of amplifier 80 is applied to thefirst accelerating electrode 121 via cable 101 and contact new of plug69. The entire video amplifier 80, shown within the dashed line, may beconveniently contained with the housing 100, and is shown in theindicated manner only as an aid to preserving clarity in the drawmg.

While specific embodiments of the invention have been described toillustrate the principles of the invention, many modifications andvariations for applying such principles in other arrangements, but whichdo not depart from the spirit of the invention, will be apparent tothose skilled in the art.

What is claimed is:

1. A device of the class described for restoring a defective cathode raytube wherein the tube has a base with contacts and includes tubeelements comprising a fila ment, cathode, control grid and screen gridwith a short circuit between the control grid and cathode and whereinthe tube base is adapted to receive a first jack transmitting signalsources for the tube element s during uncorrected operation of the tube,said device adapted to be inserted between the tube base and the firstjack therefor and comprising a second 'jack for engaging the contacts onthe tube base, a plug for connection with said first jack and circuitmeans interconnecting said second jack and said plug, said circuit meanscomprising means for connecting the filament, cathode and screengridwith the respective signal sources therefor and the signal sourcefor the control grid with the screen grid, the last named'connectionincluding an impedance;

' 2. A device according to claim 1 said circuit means further includingmeans for establishing-an external short circuit between the controlgrid and cathode.

3. A device of the class described for restoring a defective cathode raytube wherein the tube has abase with contacts and includes tube elementscomprising a filament, cathode, control grid andscreen grid with aninternally open cathode and wherein the tube base is adapted to receivea first jack transmitting signal sources for the tube elements duringuncorrected operation of the tube, said device adapted to be insertedbetween the tube 'base and the first jack therefor and comprising asecond jack for engaging the contacts on the tube base, a plug forconnection with said first jack and circuit means interconnecting saidsecond jack and said plug, said circult means comprising means forconnecting the control grid and screen grid to the respective signalsources therefor and for connecting the cathode signal source with thefilament, said circuit means further comprising transformer meansconnected between the filament and the source therefor. w v

4. A device of the class described for restoring a defective cathode raytube wherein the tube has a base with contacts and includes tubeelements comprising a filament, cathode, control grid and screen gridwith an internally open cathode and an internally open control grid andwherein'the tube base is'adapted to receive a first jack transmittingsignal sources for the tube elements during uncorre'cted'operation ofthe tube, said device adapted to be insertedbet'ween the tube' base andthe first 'jack therefor and comprising a second jack for engaging thecontacts on the tube base, a plug for connection with said first jackand circuit means interconnecting said second jack andsaid plug,saidcircuitmeans comprising means' for connecting the control grid andscreen grid with the respective signal sources therefor and forconnecting the cathode signal source with the filament and the controlgrid signal source with the screen grid, said circuit means furtherincluding transformer means connected in the circuit between thefilament and the signal source therefor.

5. A device of the class described for restoring a defective cathode raytubewherein the tube has a base with contacts and includes tube elementscomprising a filament, cathode, control grid and screen grid with adefective control grid and a partially exhausted cathode and wherein'thetube base is adapted to receive a first jack transmitting signal sourcesfor the 'tube elements during uncorrected operation of the tube, saiddevice adapted to be inserted between the tube base and the first jacktherefor andcomprising a second jack for engaging the contacts on thetube base, a plug for connection with said first jack and circuit meansinterconnecting said second jack and said plug, said circuit meanscomprising means for connecting the cathode and screen grid with therespective signal sources therefor and for connecting the screen gridwith the signal source for the control grid, said circuit meansfurtherincluding a transformer having primary and secondary windings,means for connecting the signal source for the filament with the transformer primary winding, and means for connecting the filament withthe'tr'ansformer secondary winding.

6. A device according to claim'S wherein said last named means includesan impedance.

References Cited in the file ofthis patent U IT D STATES PATENTSAckerman July 31, 1956 OTHER REFERENCES Radio and TV News, pp. 29 and 98of September 1 952,'and pp.-62 and 63 ofOctober 1952.

